There are many situations where it is necessary to set up communication links between any selected pairs of a large number of stations. This has been effected in the prior art by provision of a bus to which all the stations may be connected, and utilizing the bus to carry the information that determines whether the communication can be established. In some prior systems, much time is wasted because the decision as to the sequence with which the communication links are to be set up are made by a programmed computer or microprocessor. Furthermore, once a link is set up, it continues to operate until the message is completed so that other selected pairs may have to wait a long time before a link between them may be established, and this may be intolerable, e.g., if one of the stations includes a cathode ray tube on which data is displayed, the presentation may be interrupted for long enough periods of time to prevent it from having a real-time appearance. In other words, a minimum data rate is not guaranteed. Another disadvantage of some prior systems is that the distances over which they can operate are too short. A further problem is that there is only one chance for a message to be sent, and when it is sent, there is no assurance that it was received.